The present invention relates to a method of hardening a valve face of a poppet valve to obtain high hardness and stable quality.
A valve face of an intake or exhaust valve used in an internal combustion engine is repeatedly engaged on a valve seat, keeping airtightness, and therefore requires wear resistance. Especially, in an exhaust valve of a marine diesel engine having low quality fuel, combustion residue gets into the valve face to cause pressed traces to decrease sealability. So high hardness is required.
To satisfy the requirements, Co or Ni alloy is padded and hardened on the valve face of the poppet valve to improve wear resistance. In such a hardening method by padding, padding material is different from the valve material. So the padding material is likely to peel off or cracking occurs owing to difference in thermal expansion rate. The padding material which contains rare Co increases cost in a large-sized poppet valve.
To solve such problem, there is a way for hardening a valve face. The valve face is strongly pressed by forging, or using a press or a roller and plastically deformed, so that deformation resistance or hardness against plastic sliding increases.
However, such hardening method involves the following problems. Working rate deeply concerns hardness. As working rate increases, hardness increases, but toughness decreases to let the valve face easily broken. It is necessary to determine working rate at optimum. Working rate is determined by form of the valve face before working. If the figure is not determined at optimum, there would be variation in hardness and depth, thereby causing local wear to decrease airtightness.